Forming screens are perforated devices that are used to make apertured and embossed (unapertured) formed films. Apertured formed films are plastic films that are processed to create apertures or holes in the film. For three-dimensional films, the two most common processes are vacuum forming and hydroforming. The vacuum forming process, exemplified by references such as U.S. Pat. Nos. 3,957,414, 4,317,792 and 4,456,570, introduces a solid or molten plastic film onto a cylindrical forming screen that is rotating about a vacuum drum. In a hydroforming process, as exemplified by the reference U.S. Pat. No. 7,364,687, high pressure water jets are used to generate streams of liquid that impinge upon the film to create the three dimensional form and apertures.
In either the hydroforming or vacuum forming processes, the film is supported on a perforated structure known as a forming screen. Forming screens (also known in the art as “forming screens”), generally comprise a perforated cylinder. However, the forming screens may also take the form of a flat plate or flexible belt. The surface area of the forming screen surrounding the perforation is known as the “land” area.
One primary consideration in making metal screens is the amount of open area of the screen. With less solid area, the screens become weaker, less robust and more susceptible to breakage or distortion during use, and are more fragile to transport and handle.
To compensate for the loss of solid screen material with increased open area, it is generally desired to make a screen that has a high ratio of thickness to open area. By this is meant a comparison between the thickness of the forming screen in relation to the amount of open area in the screen. As the open area increases, the distance between the edges of adjacent openings decreases, which results in narrow land areas and a weaker screen. However, the strength of the screen can be increased by increasing the thickness of the screen.
A common problem in the art when trying to increase the thickness of the screen, especially in an electroplating process, is that the plating processes is non-specific in where the metal ions from the plating bath will be deposited as metal. Accordingly, while metal is being plated onto the outer surface to increase the thickness of the screen, metal is also being plated on the sidewalls forming the opening in the screen. Thus, as the thickness increases, the diameter of the openings in the screen will decrease. Subsequent processing steps (such as etching, drilling, laser engraving, etc.) are thus needed to increase the size of the opening of the screen back to its desired size. Therefore, it has been an objective in the art to have a process that provides screens having a relatively high thickness to open area ratio and a relatively high aspect ratio.
In an effort to address this need, plating processes have been proposed using specialized plating baths that are formulated or controlled to preferentially deposit metal so as to preferentially increase screen thickness and minimize the deposit of metal on the walls defining the aperture of the screen. Such processes are generally taught in U.S. Pat. Nos. 2,226,384, 4,383,896, 4,496,434, and 5,282,951. However, despite the improvements that these references provide, such processes have not eliminated the deposition of metal on the interior of the walls defining the opening, and thus have not eliminated the problems in the prior art to achieve the high thickness to open area ratio or a high aspect ratio.
There is a need for a process of making metal screens that have a relatively high thickness to open area ratio and a relatively high aspect ratio in a simple, controlled and low cost process.